EMD-type unit injectors are characterized by a nozzle valve body which terminates in a nozzle tip and houses a nozzle valve. The seat for the nozzle valve is formed at or near the nozzle tip and communicates with a small spray hole feed chamber, or "sac," just below the seat and within the tip. The sac has a cylindrical sidewall and a hemispherical bottom wall. The fuel is distributed through the sac under high pressure to spray holes which are several times longer than their diameter. The spray holes lead from the sac through the wall of the injector tip and into the engine chamber where the fuel is atomized.
Valves of the EMD type are further characterized by a spring seat which couples the spring to the nozzle valve. The spring holds the valve in seated, closed position until overcome by pressure of incoming fuel acting on a conical differential area of the nozzle valve. This action forces the valve in the opening direction against the bias of the spring. The spring seat and spring are carried in a spring cage stacked just above (upstream of) the nozzle valve body.
EMD-type valves are further characterized by provision of a disc type check valve carried in a check valve cage which is stacked just above or upstream of the spring cage.
The spring cage, check valve cage, and nozzle valve body are stacked coaxially one above the other within the injector housing-nut. The stack length of the nozzle valve body slightly exceeds the combined stack lengths of the check valve cage and the spring cage. The injector housing-nut houses the stacked components that are at the injection end of the injector. The housing-nut is fixed in the head of the engine and extends through it. On the exterior side, the housing-nut is threaded to and acts as an extension of the main housing of the pump-injection unit.
In today's diesel engine operating environment, the general public is reminded daily about the health effects of exhaust emissions. As a result, the government is relentlessly reducing the levels of permissible smoke and hydrocarbons emitted from the engine exhaust. There is a great need for improvements to meet the requirements of ever-increasing government restrictions, particularly for improvements in EMD-type locomotive fuel injectors, a type already widely used and whose use can be widely supported by existing networks of rebuilders as well as original equipment manufacturers.
It is universally recognized today in the fuel injection and diesel engine industries that reducing the sac volume of closed-type inwardly-opening nozzles reduces engine exhaust smoke and hydrocarbon emissions, all other factors being equal. However, reducing the sac volume of a EMD-type injection nozzle is not a simple matter. Maintaining the integrity of the nozzle durability characteristics is the primary consideration of a product, as performance improvement at the expense of reliability is totally unacceptable. In addition, reducing the sac volume must not compromise the optimum relationship of the nozzle spray hole length with respect to the hole diameter. The present invention enables these durability and spray hole requirements to be maintained while still reducing the sac volume to thereby achieve improvements in exhaust emissions.
The rate at which the nozzle valve closes is also known to have an influence on the quality of the fuel spray issuing from the nozzle at the very end of injection. If the nozzle valve closes slowly, the fuel that leaves the sac during the closing phase is replaced by fuel continuing to flow past the nozzle valve seat into the sac. If the nozzle valve closes rapidly, less fuel will flow into the sac, but in addition, the rapid valve displacement into the valve seat desirably gives added force to dispel the fuel from the sac through the orifices into the engine combustion chamber thus leaving little or no fuel in the sac to be drawn out in the late stages of the engine expansion stroke. Fuel drawn out of the sac during the late stages of expansion contributes greatly to hydrocarbon emissions and carboning of the nozzle tip.
There are several means by which increase of the nozzle valve closing rate can be accomplished. One would be to increase the nozzle opening pressure above the present specification level. However, this would tend to cause irregular injection in the low-speed part load range and idle. It would also increase the nozzle spring stress causing increased fall-off in opening pressure over time or, in some cases, resulting in spring failure. Changing the valve/seat diameter ratio would have similar effects.
Another means would be to make the nozzle valve opening pressure the same for all injectors. The specification for nozzle opening pressure of reconditioned injectors is 2800 to 3400 psi. During engine operation, the nozzle spring "sets," edges wear, and the spring length shortens a little so that the nozzle opening pressure decreases. Therefore, when rebuilding injectors reusing old springs, the opening pressure would tend to be toward the minimum specification level for most injectors. When using new springs also, there are slight differences in free length, wire diameter and effective coils; add to these, variation in spring cage length, all within the respective part specification tolerances, of course, and we have nozzle opening pressure variation between injectors being quite broad. All involved part specification tolerances are taken into consideration when the nozzle opening pressure specification is established, and this is the reason the nozzle opening pressure specification is so broad. Therefore, to obtain the maximum level of performance from an injector, it is preferable to set the opening pressure at the maximum level of the specification. This cannot be done with present EMD-type injectors. The present invention makes it possible to achieve this objective.
Still other improvements to improve engine exhaust smoke and hydrocarbon emission performance of EMD-type locomotive fuel injectors may be provided by the invention. These will appear in the following description, from which the improvements discussed above also will be more fully understood.